1. Field of the Invention
The present invention generally relates to a solid oxide fuel cell and a manufacturing method thereof and, more particularly, to a solid oxide fuel cell comprising a nano-structured electrode with a metal support operating at intermediate temperature and a manufacturing method thereof.
2. Description of the Prior Art
The solid oxide fuel cell (SOFC) is an electrochemical power generation device, in which oxygen and hydrogen are used for power generation so as to achieve high power generation efficiency with low pollution. There are numerous reports on the electrolyte, the anode and the cathode of an solid oxide fuel cell, such as Appleby, “Fuel cell technology: Status and future prospects,” Energy, 21, 521, 1996; Singhal, “Science and technology of solid-oxide fuel cells,” MRS Bulletin, 25, 16, 2000; Williams, “Status of solid oxide fuel cell development and commercialization in the U.S.,” Proceedings of 6th International Symposium on Solid Oxide Fuel Cells (SOFC VI), Honolulu, Hi., 3, 1999; and Hujismans et al., “Intermediate temperature SOFC—a promise for the 21th century,” J. Power Sources, 71, 107, 1998). Generally, the electrolyte is made of yttria-stabilized zirconia (YSZ), the anode is made of a cermet (Ni/YSZ) composed of nickel and yttria-stabilized zirconia (YSZ), and the cathode is made of conductive lanthanum strontium-doped manganite (LSM, LaMnO3) with a perovskite structure.
However, since yttria-stabilized zirconia (YSZ) exhibits sufficient ion conductivity only at high temperatures within a range from 900 to 1000° C., the solid oxide fuel cell made of high-cost materials is thus not widely used.
Therefore, in the prior art, a thinner yttria-stabilized zirconia (YSZ) electrolyte layer (about 5 μm) is provided to reduce the resistance and loss under the working temperature lowerer than 900° C. Alternatively, an electrolyte (made of, for example, lanthanum strontium gallate magnesite (LaGaO3), LSGM) with high ion conductivity can be used to manufacture a solid oxide fuel cell that works at intermediate temperature (600 to 800° C.) with lower manufacturing cost. As the working temperature is reduced, the reliability and duration of the solid oxide fuel cell can be improved so that it is helpful to make the solid oxide fuel cell more acceptably used in home and car applications.
However, when the working temperature of the solid oxide fuel cell is lowered to about 600° C., a thinner yttria-stabilized zirconia (YSZ) electrolyte layer (about 5 μm) will not have enough ion conductivity to satisfy the low resistance loss requirement. Therefore, other electrolyte materials such as gadolinium doped ceria (GDC) or lanthanum strontium gallate magnesite (LSGM) with high ion conductivity are required
Moreover, as the temperature decreases, electrochemical activities at the cathode and anode decrease, and polarization resistances at the cathode and anode increase with a larger energy loss. Therefore, new materials for the cathode (such as lanthanum strontium cobalt ferrite (LSCF, La0.6Sr0.4Co0.2Fe0.8O3)) and the anode (such as a mixture (GDC/Ni) composed of nickel and gadolinium doped ceria (GDC) or a mixture (LDC/Ni) composed of nickel and lanthanum doped ceria (LDC)) are required. Moreover, in the prior art, the cathode and the anode are mostly micron-structured, which should be improved to be nano-structured so as to increase the number of tri-phase boundaries (TPB) to improve the electrochemical activities at the cathode and the anode to reduce energy loss.
For the anode structure, in Virkar's “Low-temperature anode-supported high power density solid oxide fuel cells with nano-structured electrodes,” Fuel Cell Annual Report, 111, 2003, a Ni/YSZ cermet as the anode of a solid oxide fuel cell is disclosed with a thin layer of smaller pores and a thick layer of larger pores. The diameters of the smaller pores should be as small as possible to increase the number of tri-phase boundaries (TPB). However, Virkar fails to disclose how to manufacture the thin layer with nano structure in that report.
Moreover, Wang also discloses, in “Influence of size of NiO on the electrochemical properties for SOFC anode,” Chemical Journal of Chinese Universities, a mixture of nano NiO and micron YSZ is press-formed and reduced by hydrogen to obtain a cermet anode with increased tri-phase boundaries (TPB) and reduced electrode energy loss. However, Wang also fails to disclose how to make a nano-structured anode in that paper.
For the cathode structure, in Liu's “Nano-structured and functionally graded cathodes for intermediate temperature solid oxide fuel cells,” J. Power Sources, 138, 194, 2004, a nano and functionally graded structured cathode is manufactured by combustion chemical vapor-phase deposition. Since the number of electrochemical reaction sites or tri-phase boundaries (TPB) at the cathode is increased, the polarization and ohmic resistances are lowered to reduce the energy loss.
For the electrolyte, as the electrolyte thickness increases, the internal resistance of the solid oxide fuel cell increases to cause larger energy loss and smaller output power. More particularly, when the working temperature of the solid oxide fuel cell is below 700° C., the energy loss due to electrolyte resistance becomes dominant. Therefore, the electrolyte thickness has to be reduced or the ion conductivity in the electrolyte has to be enhanced so as to improve the output power delivered by the cell.
Generally, the solid oxide fuel cell can be manufactured by (1) chemical vapor-phase deposition (CVD) (2) electrochemical vapor-phase deposition (3) sol-gel (4) strip casting (5) silk screen printing (6) physical vapor-phase deposition (7) spin coating and (8) plasma spray. There are two methods to perform plasma spray: atmospheric plasma spray and vacuum plasma spray. In the above manufacturing methods, strip casting, silk screen printing and spin coating require plural high-temperature sintering processes, while chemical vapor-phase deposition (CVD), electrochemical vapor-phase deposition, sol-gel, physical vapor-phase deposition and plasma spray can be used to manufacture the solid oxide fuel cell without high-temperature sintering processes.
In the manufacturing methods requiring high-temperature sintering processes, it often leads to warping and cracks in the components of the solid oxide fuel cell during high-temperature sintering.
Moreover, high-temperature sintering is often used to obtain the dense electrolyte layer and improve the contact between the electrolyte layer and the electrode layer, but it also causes the porous electrode layer to become denser and less mass transfer. Moreover, high-temperature sintering process often results in chemical reactions between the electrolyte layer and the electrode layer, those reactions are often unfavorable to the cell performances and occur. For example, the lanthanum strontium gallate magnesite (LSGM) electrolyte layer reacts at high temperatures with nickel in the anode layer to produce an insulating lanthanum nickel oxide (LaNiO3) layer and to increase the internal resistance of the solid oxide fuel cell. (See Zhang et al., “Interface reactions in the NiO-SDC-LSGM system,” Solid State Ionics, 139, 145, 2001). Furthermore, high-temperature sintering causes the nano-particles in the anode, the cathode and the electrolyte to aggregate and grow into sub-micron particle or micron-particles.
U.S. Patent Appl. No. 2007/0009784 discloses an intermediate temperature solid oxide fuel cell manufactured by high-temperature sintering. The anode is formed of a mixture (LDC/Ni) composed of nickel and lanthanum doped ceria (LDC, La0.4Ce0.6O2); the electrolyte is formed of lanthanum strontium gallate magnesite (LSGM); and the cathode is formed of an interlayer comprised of lanthanum strontium gallate magnesite (LSGM) and lanthanum strontium cobalt ferrite (LSCF) with 50%:50% volumetric ratio and a current collecting layer comprised of lanthanum strontium cobalt ferrite (LSCF).
In order to prevent lanthanum strontium gallate magnesite (LSGM) electrolyte from reacting with nickel particles in the anode layer to produce insulating lanthanum nickel oxide (LaNiO3) at high temperatures such as 1200 to 1300° C. for sintering anode and 1100° C. for sintering cathode, an isolation layer formed of lanthanum doped ceria (LDC) is added between the anode and the electrolyte.
However, when the thickness of lanthanum strontium gallate magnesite (LSGM) electrolyte is smaller than 20 μm, cobalt (Co) particles in lanthanum strontium cobalt ferrite (LSCF) cathode diffuse into the lanthanum strontium gallate magnesite (LSGM) electrolyte at high temperatures to worsen the electron insulation of this electrolyte and cause electron transport and internal leakage in the solid oxide fuel cell. As a result, the open-circuit voltage is smaller than 1 volt. In other words, it is inevitable that the manufacturing methods requiring high-temperature sintering are problematic of element diffusions and reactions at high temperatures.
Among the manufacturing methods without high-temperature sintering, the atmospheric plasma spray is very potential and has attracted lots of attention. More particularly, the plasma flame of atmospheric plasma spray is capable of heating up the injected powders to be melted or semi-melted. The melted or semi-melted powders are cooled down and turned into a film instantly after they bombard the substrate. In this method, chemical reactions (for example, to produce insulating lanthanum nickel oxide (LaNiO3)) that are unfavorable to the cell performances can be avoided, as disclosed in Hui et al., “Thermal plasma spraying for SOFCs: Applications, potential advantages, and challenges,” J. Power Sources, 170, 308, 2007.
U.S. Pat. No. 7,141,271 discloses a vacuum plasma spray method for producing a solid oxide fuel cell, wherein a solid electrolyte layer deposited on an electrode inside a coating chamber at a vacuum less than 15 mbar is gas-tight. The powders injected into the plasma spray gun are micron powders smaller than 10 μm. However, the vacuum plasma spray is more complicated, expensive and less efficient in mass production.
Moreover, U.S. Patent Appl. No. 2004/0018409 discloses a solid oxide fuel cell manufactured by dual-gas atmospheric plasma spray with low voltage (lower than 70V) and high current (larger than 700 A). In this patent, when the thickness of the lanthanum strontium gallate magnesite (LSGM) electrolyte is larger than 60 μm, the open-circuit voltage (OCV) is larger than 1V. Since the plasma arc root at the anode nozzle of plasma spray gun moves with the plasma gas stream to cause voltage variation ΔV of the plasma spray gun. Therefore, the atmospheric plasma spray with a gun working at low voltage and large current exhibits a relatively large voltage variation ratio ΔV/V which leads to an unstable powder heating and an unreliable coating.
Moreover, in the low-voltage high-current dual-gas atmospheric plasma spray, the shorter plasma arc leads to a shorter heating time and a poorer thermal heating efficiency of powders. Moreover, the high current results in the serious erosions of cathode and anode electrodes of atmospheric plasma spray gun. The cathode and the anode are updated more frequently and the cost of manufacturing solid oxide fuel cells increases.
In U.S. Patent Appl. No. 2004/0018409, the micron powder clusters for plasma spray are formed by aggregating powders smaller than 100 nm with a polyvinyl alcohol (PVA) binder. The PVA binder is then removed by conventional heating processes to acquire sintered porous nano-structured micron powder clusters. These nano-structured micron powder clusters formed by complicated processes in this patent increase the cost of manufacturing the solid oxide fuel cell. Moreover, to increase the surfaces of these micron powder clusters for heating by plasma flame, these powder clusters are often formed in a hollow structure that costs more.